[llvm] [LIT][Cygwin] Skip pre-check for existence in mkdir-p (PR #163948)

[Alexander Richardson via llvm-commits]

arichardson wrote:

>From what I've read pathlib.Path handles long windows paths correctly, so maybe the best solution would be to migrate callers of these helpers towards pathlib?

https://github.com/llvm/llvm-project/pull/163948